Plant MAPK Cascade Under Stress

Due to global warming and population growth, plants need to rescue themselves, especially in unfavorable environments, to fulfill food requirements because they are sessile organisms. Stress signal sensing is a crucial step that determines the appropriate response which, ultimately, determines the survival of plants. As important signaling modules in eukaryotes, plant mitogen-activated protein kinase (MAPK) cascades play a key role in regulating responses to the following four major environmental stresses: high salinity, drought, extreme temperature and insect and pathogen infections. MAPK cascades are involved in responses to these environmental stresses by regulating the expression of related genes, plant hormone production and crosstalk with other environmental stresses. In this review, we describe recent major studies investigating MAPK-mediated environmental stress responses. We also highlight the diverse function of MAPK cascades in environmental stress. These findings help us understand the regulatory network of MAPKs under environmental stress and provide another strategy to improve stress resistance in crops to ensure food security.

MAPK in drought

Drought stress affecting food productivity has become a troublesome problem worldwide. Drought stress is a complex stress that causes multidimensional changes, such as physiological processes, molecular mechanisms and morphological adjustments. Moreover, the effect caused by drought stress differs across developmental stages and plant species. As a major signal transducer, the MAPK cascade plays a vital role in drought stress, generally by responding to ABA and regulating ROS production. Moreover, several WRKY transcription factors have been identified as substrates of the MAPK cascade in drought stress. According to RNA-Seq analyses, numerous components of MAPK cascades have been reported to respond to drought in crops. These findings highlight the importance of MAPKs in drought, but knowledge regarding their biological functions under drought stress is limited. Further studies should expand efforts to uncover their biological functions in drought stress. To date, the substrate of the MAPK cascade under drought stress has been identified, but the upstream MAPK cascade in drought stress remains unknown. Further studies should exert efforts to identify the RLKs and receptor-like proteins (RLPs) upstream of MAP3K and their function in drought stress.

MAPK in biotic stress

Potential pathogens exist in the air and soil and consistently threaten plant adaption and crop productivity. Using chemical pesticides in planting areas is the most common strategy, but this method dramatically destroys the balance between humans and ecology. Cultivating resistant crops has become the most effective and environmentally friendly way to address this serious problem. During a long period of plant–pathogen interactions, plants have evolved sophisticated immune systems to prevent pathogens from invading. The MAPK cascade plays a critical role in the plant defense response. MPK3, MPK4 and MPK6 are activated after pathogen perception to induce an early defense response. MPK3, MPK4 and MPK6 regulate plant disease resistance by regulating phytoalexin and phytohormone biosynthesis in biotic stress and activating downstream substrates, which play a vital role in the early plant defense response. The upstream of MPK3, MPK4 and MPK6 in biotic stress has already been identified. Other studies also showed the function of other MAPK cascade members in biotic stress.

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Approximately 20 MAPKs have been identified in Arabidopsis, but only three MAPKs (MPK3, MPK4 and MPK6) are well studied in environmental stress. A few advances have revealed the functions of 17 other MAPKs in environmental stress. Hence, the regulatory network of the MAPK cascade seems to be generally single. With the development of high-throughput phospho-proteomics analysis, many transcription factors, enzymes and proteins have been shown to be candidate substrates of MAPKs. Further studies should exert efforts to reveal the function of the 17 other MAPKs in environmental stresses. More importantly, identifying novel substrates of MAPKs is essential to enrich the current understanding of MAPK regulation under environmental stress.

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